Sheet material such as corrugated stock is typically discharged single file and along a plane from a finishing machine in which the sheets are printed, die cut, creased, perforated or otherwise treated by the finishing machine. The sheets may be rectangular or of other die cut configurations, but are typically flat and unfolded when leaving the finishing machine. Other processes downstream of the finishing machine discharge are used to form the sheet material into desired configurations. One such configuration is a flat partially folded box shape, the folding process of which being exemplified in FIG. 1 of the drawings.
A problem has been experienced in transporting single sheets of sheet material such as corrugated box stock, downstream of finishing machines. The individual sheets may be of various sizes and configurations, and are typically quite light weight. A conveyor with lugs for engaging trailing edges of sheet materials is typically inadequate for sheet material handling for at least three reasons.
Firstly, the variety of sizes and shapes of sheet material being discharged from the finishing machine dictates that the conveyor lugs be adjustable. Adjustable lugs on conveyor belts are not practical.
Secondly, there is a high probability that the engaged edges of the sheets would be damaged by engagement with the lugs. Damaged sheet material is not acceptable.
Third, a conveyor lug engaging only the trailing edges of individual sheets does not permit effective control of the sheet during transport. The sheets typically have large surface areas and are light weight, and will often lift up from the conveying surface, especially if the conveyor is operating at speeds matching the production rate of the finishing machine. This creates a sheet handling problem, increases the probability of damage to the sheets, and increases the likelihood of disruption in downstream handling equipment.
Conveyors used without trailing edge engaging lugs would be more desirable, considering there would be no need for adjustments for sheet size or shape, and no trailing edge damage would be likely. However even rough surface belting is not adequate to assure a firm grip on the sheets and slippage may be expected. Also the sheet materials would still tend to periodically lift from the belt surface.
A solution to the above problems has been to incorporate two belt conveyors with the working flight of one conveyor engaging the underside of the sheet material and the working flight of the other conveyor engaging the top surface of the sheet. While this provides more effective grip without requiring conveyor lugs, and control of the sheet materials is improved, slippage is still possible. Further, the upper belt will often smudge or streak ink applied by the finishing machine.
A need therefor remains for a sheet material conveyor which will provide positive control of the sheet materials, avoid damage to the sheet surfaces and edges, and that will not permit the sheet material to lift up from the conveying surface during transport. The present invention fills these needs, as will be understood from the following description.